A rotating electric machine for a vehicle has been known. Generally, a rotating electric machine has a casing that houses a stator and a rotor. The casing may be clamped between a first frame and a second frame in an axial direction. The first frame and the second frame may be fastened together by a through bolt. For example, in a starter motor disclosed in a patent document (JP 2003-184711 A), the first frame and the second frame have a first flange and a second flange, respectively. Both the first and second flanges protrude outwardly in a radial direction of a rotor and are fastened together by a through bolt.
In the starter motor of the patent document, since the head of the through bolt is fully exposed on a side surface of the first flange of the first frame that is opposite to the casing, the head of the through bolt may interfere with other components of the starter motor.
As a solution, a counterbore is formed in the first flange so that all or part of the head of the through bolt is hidden within the counterbore. As a result, interference of the head with other components can be avoided. Conventionally, the counterbore is a blind hole having a circular cross-sectional shape. Further, in a cross-sectional plane on which an axial direction of the rotor exists, a fillet curve between a bottom surface and an inner side wall of the counterbore has a curvature of constant radius.
However, as a result of the above configuration, stress may be focused locally on a portion of the counterbore due to a fastening force by the through bolt in a direction from the first flange toward the second flange. Typically, the portion to which the fastening force is focused locally is an area of a fillet curve between the bottom surface and a side wall surface of the counterbore and is positioned on a virtual circle that is concentric with a rotor and passes through an axis of the counterbore.